Direct Driver for LED lamp are popular for its low cost, low EMI and high efficiency. However, they all suffer from limited input voltage range. As voltage is lowered, the LED current will decrease and the light output will dim. What is particularly troubling about this phenomena is that the normal voltage ripple at the output of the rectifier will cause the light output to flicker at twice the input line voltage frequency (e.g., AC voltage) when input voltage becomes low. Although this is not consciously perceptible to most people, it can have deleterious health effects including eye strain, headache, and in the worst case, it can trigger epileptic seizures in people who are prone to those seizures.
FIG. 1 shows one of the conventional direct driver for an LED lamp. The LED lamp is divided into a major string 10 and a minor string 11. The major string 10 contains a fixed number of LED diodes while the driver constantly changes the number of electrically connected LED diodes in the minor string 11 to provide optimal light output.
As the input voltage is lowered, less and less of LED diodes the of minor string 11 are configured in series with the major string 10. Low input voltage operation defined occurs when the LED diodes of the minor string 11 drops to zero, the overall number of the LED diodes cannot be lowered any more. As further decreasing voltage of the input voltage result in lower LED current and subsequent lower luminous output power. If, as well as being lower than normal, the input voltage has an AC component, then the light output of the LED string will show the same AC component.
Therefore, there is a need for an approach to provide a scheme or means to have an anti-flicker circuit for an LED direct driver especially for a low input voltage operation.